In order to elucidate the mechanism involved in the onset of parturition in the rat, we will study the sequence of hormonal interplay between the feto-placental unit and the maternal endocrine-reproductive system. The study will be focused on investigating how an increase in ACTH in the fetus near term causes the uterus to contract and expel the fetus. Special attention will be paid to the age dependent difference in the fetal adrenal response to ACTH and to the site of action of the parturition-inducing factor(s) originated from the mature feto-placental unit.